From: Roman Penyaev <rpenyaev@suse.de>
To: Davidlohr Bueso <dbueso@suse.de>, Jason Baron <jbaron@akamai.com>,
Al Viro <viro@zeniv.linux.org.uk>,
Andrew Morton <akpm@linux-foundation.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/1] epoll: remove wrong assert that ep_poll_callback is always called with irqs off
Date: Tue, 08 Jan 2019 13:42:25 +0100 [thread overview]
Message-ID: <f582f40b9697dced81d16b3b35a4b071@suse.de> (raw)
In-Reply-To: <20190108100121.20247-1-rpenyaev@suse.de>
On 2019-01-08 11:01, Roman Penyaev wrote:
> That was wrong assumption that all drivers disable irqs before waking
> up
> a wait queue. Even assert line is removed the whole logic stays
> correct:
> epoll always locks rwlock with irqs disabled and by itself does not
> call
> from interrupts, thus it is up to driver how to call wake_up_locked(),
> because if driver does not handle any interrupts (like fuse in the the
> report) of course it is safe on its side to take a simple spin_lock.
This is wrong and can lead to dead lock: we always call read_lock(),
caller
can call us with irqs enabled. Another driver, which also calls
ep_poll_callback(), can be called from interrupt context (irqs disabled)
thus it can interrupt the one who does not disable irqs. Even we take
a read_lock() (which should be fine to interrupt), write_lock(), which
comes in the middle, can cause a dead lock:
#CPU0 #CPU1
task: task: irq:
spin_lock(&wq1->lock);
ep_poll_callback():
read_lock(&ep->lock)
....
write_lock_irq(&ep->lock) ....
#waits reads .... >>>>>>>>>>>>>> IRQ CPU1
spin_lock_irqsave(&wq2->lock)
ep_poll_callback():
read_lock(&ep->lock);
# to avoid write
starve should
# wait writer to
finish, thus
# dead lock
What we can do:
a) disable irqs if we are not in interrupt.
b) revert the patch completely.
David, is it really crucial in terms of performance to avoid double
local_irq_save() on Xen on this ep_poll_callback() hot path?
For example why not to do the following:
if (!in_interrupt())
local_irq_save(flags);
read_lock(ep->lock);
with huge comment explaining performance number.
Or just give up and simply revert the original patch completely
and always call read_lock_irqsave().
--
Roman
next prev parent reply other threads:[~2019-01-08 12:42 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-01-08 10:01 [PATCH 1/1] epoll: remove wrong assert that ep_poll_callback is always called with irqs off Roman Penyaev
2019-01-08 12:42 ` Roman Penyaev [this message]
2019-01-08 15:16 ` Davidlohr Bueso
2019-01-08 16:07 ` Roman Penyaev
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